Accelerated Upgrade to Anaerobic Digestion Facility Wins New Business for City of Cortland

Cortland New York Wastewater Treatment Plant digester covers and tanks upgrade

When a large dairy expressed interest in placing a new Greek yogurt plant in an industrial park in Cortland, New York, the community improved its chance to secure the plant and provide future economic growth by upgrading the City’s wastewater treatment facility so it could offer no-cost trucked waste disposal for the dairy facility. Upgrading the WWTP’s anaerobic digestion was key to accommodating the extra waste and generating energy for the plant.

Before the upgrade, the facility had limited anaerobic digester capacity. Though the 1940s-era digesters had been upgraded once, they were inadequate for the current needs of the facility, let alone the additional output from the Greek yogurt plant.

One of the significant issues driving the upgrade was that the acid whey from the Greek yogurt process could not be accommodated by the current wastewater treatment facility. The municipality needed a quick and forward-thinking solution for the plant that serves the communities of Cortland, Homer, McGraw, and a portion of Cortlandville, New York.

The $27-million plant upgrade was partially funded by a $13.5-million bond from EPA’s Clean Water State Revolving Fund and $2.3 million in grants.

Meeting Environmental Benchmarks

The wastewater facility is far and away the largest greenhouse gas (GHG) producer within the Cortland city government, said Bruce Adams, Superintendent of the Cortland Wastewater Treatment Facility.

The combined heat and power project at the heart of our relationship with the dairy businesses will reduce our GHG to a small fraction of what it was. The next phase CHP project promises to reduce the carbon footprint to near zero.

The upgrade was also driven by stricter environmental requirements. The wastewater treatment facility’s location along the Tioughnioga River in the Chesapeake Bay watershed also required compliance with changing Chesapeake Bay nutrient discharge regulations.

Equipment Selection – Low Maintenance, Easy Installation

The municipality had initially planned a new, large digester to accommodate the trucked waste, as well as a modest upgrade to their existing digester facilities. However, the time required for this option caused the facility to rethink the design of the project, and they decided to do a thorough upgrade to the existing digesters to more quickly accommodate the needs of the new yogurt plant.

The new plan replaced the aging carbon steel fixed covers with new covers made of 304 stainless steel, changed the gas storage from a fiberglass floating cover to a DuoSphere™ gas holder, and converted the secondary digester to a third primary digester. These upgrades increased the plant’s capacity so it could accommodate dairy waste and give the facility staff breathing room to plan the new digester and associated CHP equipment to be completed in a later phase.


The existing gas mixing systems on the primary digesters were only marginally effective, even when they were working properly. The plant had issues with throughput, volatile solids reduction, and gas production. It was clear that the plant needed to improve their mixing to meet the project goals.

WesTech Extreme Duty Sludge Mixer at Cortland New York wastewater treatment plant The facility had seen and wanted a gas piston-type system for the mixer because the design had no moving parts in the tank. But this required a costly retrofit and would have taken valuable building space for the compressors.

The facility decided to install WesTech Extreme Duty™ Sludge Mixers that saved space, assured reliability, and helped the project schedule to stay on track. The internal mixers were easily mounted at the center of each of the two fixed covers. The two external units mounted to the outside of the DuoSphere digester.


The existing steel covers had corroded badly and needed to be replaced. To meet the schedule, two stainless steel digester covers were provided to replace the existing ones. Stainless steel covers are faster to install as they require less welding than carbon steel covers, no blasting, and no painting.

WesTech DuoSphere Dual Membrane digester cover at Cortland New York wastewater treatment plant

The installation of the stainless steel covers was particularly well suited to the Cortland plant and its project schedule. Steel covers eliminate priming and painting and the need to periodically inspect the interior coatings. The shop welding of alternate “pie slice” pieces allowed the covers to be quickly installed.

To capture the increased gas produced by the added whey, the DuoSphere dual membrane cover was used to replace the floating cover. This would give the facility significantly more storage and flexibility when it implemented cogeneration in the next phase of the project. The ability to run at a higher operating pressure than they had with the concrete-ballasted rigid fiberglass gas holder was also a plus.

The DuoSphere was the first cover placed, requiring just three weeks for installation (much shorter than the steel covers) and allowing this newly-covered and mixed digester to be started up much sooner than the others.

Heat Exchanger

The tube-in-tube heat exchanger was installed to preheat the whey before it is sent to the digesters. This feedstock is trucked in from the dairy processing facility, and the heat exchanger ensures that it stays in a liquid state for processing. The heating requirement is more significant in the winter, but the heat exchanger system is designed to accommodate these seasonal variations.

Digester heat exchanger Cortland New York wastewater treatment plant

Executing Quickly With Exceptional Teamwork

The timeline was aggressive, with the purchase order created in March and the equipment delivered to the site in early June. The tight schedule forced the team to overcome any design obstacles that threatened delay. With the time constraints, options were chosen that would work well with the equipment that was already in place at the facility.

For example, for the new mixer, the client had originally wanted a Bubble Gun System because there were no moving parts in the mixer. But this requires a costly retrofit for existing mixers because the equipment must be put in the tank. The decision was made to install the draft tube impellor-style mixer, Extreme Duty Sludge Mixer, which meant the installation was much simpler as the mixer sits on the outside of the tank.

One of the things that sets this project apart from others is how well the project team, client, contractors, manufacturer, and engineers cooperated and adapted to the various needs of the project under the tight timeline.

All partners worked in concert, putting the success of the project ahead of other considerations, Adams said. Having partners that were nimble and responsive was very important to the project’s success.

Sustainability, Cogeneration, and Economic Growth

The Cortland WWTP successfully transformed a 75-year old digester complex into a facility with a 50% greater hydraulic capacity and the ability to treat this new, unique waste stream in a sustainable way. It is now reliably able to treat a maximum of 9 MGD with an average of 5.2 MGD.

The project is a timely response from the municipality to the challenge of climate change and shows potential residents and businesses that the City is proactive regarding environmental stewardship.

The digester facility has a much greater capacity because of the upgrades, particularly with the mixing improvements, and this will benefit the plant for many years to come. Carbon steel equipment was replaced with long-lasting stainless steel to avoid the regular expenses, downtime, and hazards of inspection, repainting and replacement.

This project quickly gave the WWTP the ability to provide no-cost trucked waste disposal for the new dairy plant. This improves the city’s competitive economic position, allows for future opportunities, and strengthens the local economy.

Additionally, the update also gave the City of Cortland more control over operational costs of its most energy-intensive endeavor: wastewater treatment.

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